Here is a summary of the most prevalent diseases in corn, soybeans, and grain sorghum in Kansas during the 2017 growing season.
Corn diseases
If one thing can be counted on in Kansas corn production, when it comes to diseases, every year is different. One trend that continues is that southern rust is arriving earlier each year. Historically, southern rust arrived in mid-July to early August, but in recent years, it has arrived in late June to mid-July. In years with late-planted fields, this disease can easily result in 10 – 30 percent yield losses if not treated with a fungicide. By the end of the year, southern rust could be found in nearly every corn field. However, fewer than 10 percent of the fields had levels severe enough and early enough to warrant spraying. Some fields that should have received treatment were not sprayed. This was due either to a lack of scouting or growers not wanting to increase production costs while corn prices remained low.
Figure 1. Southern rust of corn. Pustules typically form only on the upper leaf surface (panel A). Photo courtesy of the Crop Protection Network.
Gray leaf spot levels in 2017 were down from the two previous years, but closer to the long-term average. Only fields planted to more susceptible hybrids required a fungicide application.
Active scouting for corn bacterial streak, our most recently discovered foliar disease, resulted in 21 new counties testing positive for the disease, bringing the total number of known infested counties to 38 in Kansas. It remains to be determined if the disease significantly affects yield.
Figure 2. Corn bacterial streak. Photo courtesy of Dr. Tamra Jackson-Ziems, University of Nebraska – Lincoln.
While 2016 was a good year for Diplodia ear rot in the state, lack of rain at and shortly after pollination in 2017 resulted in a substantial reduction of this disease across the state, except for southwest Kansas where Diplodia levels were surprisingly high.
While Aspergillus ear rot, the cause of aflatoxin problems, was present at its highest levels since 2012, actual levels of aflatoxin were surprisingly low. The highest levels occurred early in harvest and then steadily decreased as harvest progressed. A second mycotoxin, fumonisin, associated with Fusarium ear rot, became an issue for growers in the Texas Panhandle. While found in some southwest Kansas cornfields, toxin levels were generally well below the 60 parts per million (ppm) advisory level set for ruminant animals.
Figure 3. The starburst pattern on kernels is diagnostic for Fusarium ear rot. Photo by Doug Jardine, K-State Research and Extension.
Losses due to stalk rots were average in 2017. Fusarium stalk rot was by far the predominant disease found in grower fields, but charcoal rot, Diplodia stalk rot and anthracnose stalk rot were all reported at low levels around the state.
Sorghum diseases
The 2017 Kansas sorghum crop was generally healthy. Unlike 2016 when sooty stripe and rust could readily be found, foliar diseases were kept to a minimum. Gray leaf spot did cause some issues in at least one field in Cloud County.
Figure 4. Gray leaf spot of grain sorghum. Photo provided by Doug Jardine, K-State Research and Extension.
One interesting note is that sorghum downy mildew was reported in northwest Kansas, where unusually high rainfall amounts triggered the disease in a few fields with chronic wet spots.
The most significant sorghum disease in 2017 was Fusarium stalk rot. Fusarium is favored by wetter springs followed by a dry summer, and then more rain as harvest approaches. This scenario occurred in many places in the state. Late-season lodging due to Fusarium stalk rot was reported from many locations.
Soybean diseases
The most serious soybean disease problem in Kansas in 2017 was charcoal rot. This disease occurred primarily in southeast, east central, and northeast Kansas where little rain fell after mid-August.
Interestingly, the second most common disease problem in 2017 was Phytophthora root rot. Phytophthora is associated with wet soils and while it was dry late in the season, plenty of rain fell early resulting in the higher-than-normal levels of Phytophthora. The disease was most common in fields planted to susceptible varieties.
Figure 5. Stem browning caused by Phytophthora root rot. Photo courtesy of the University of Wisconsin.
The incidence of Sudden Death Syndrome continues to decline with the usage of ILeVO seed treatment and the planting of more tolerant varieties. While no new counties were recorded as being positive for soybean cyst nematode (SCN), numerous soil samples were received with egg numbers high enough to be causing economic losses. Growers should continue to soil test for SCN and rotate their varieties.
Other soybean diseases occurring in 2017 included frogeye leaf spot, Cercospora leaf blight/purple seed stain, Phomopsis pod and stem blight, anthracnose, and two virus diseases, soybean vein necrosis virus and tobacco ringspot virus. These diseases resulted in economic yield loss in a few isolated fields.
The 2018 growing season will be a new year with yet-to-be-determined disease issues. Weather, as usual, will determine which diseases thrive. Soybean growers are urged to continue to use planting time seed treatments and to select resistant/tolerant varieties in fields with a history of SCN, Sudden Death Syndrome, and Phytophthora root rot.
Doug Jardine, Extension Plant Pathology
jardine@ksu.edu
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